Molybdenum is an essential trace element for all forms of life, and over 50 molybdoenzymes are known that catalyze oxidation-reduction reactions that are essential in the metabolism of carbon, nitrogen and sulfur. In humans molybdoenzymes play physiologically vital roles in the oxidation of sulfite to sulfate (sulfite oxidase) and in certain aspects of purine metabolism (xanthine oxidase, xanthine dehydrogenase). Fatal simultaneous deficiencies in the activities of these enzymes due to an inborn deficiency of a "molybdenum cofactor" have been well documented in children. Point defects in the sulfite oxidase protein itself can also produce the severe neurological symptoms of sulfite oxidase deficiency. These symptoms include dislocated ocular lenses, mental retardation, and, in severe cases, early death. Development of methods to clone and express human sulfite oxidase has revealed several different clinical point mutations that result in isolated sulfite oxidase deficiency. The X-ray structure of the highly homologous chicken liver sulfite oxidase provides a molecular basis for interpreting the fatal point mutations of the human enzyme. The research proposed here addresses the fundamental properties of sulfite oxidase and other molybdoenzymes by an integrated program of biophysical, biochemical and model compound studies. Emphasis will be given to the use of variable frequency pulsed electron paramagnetic resonance (EPR) techniques, especially electron spin echo envelope modulation (ESEEM) and pulsed electron-nuclear double resonance (ENDOR) spectroscopies, to probe in detail the surroundings of the molybdenum active site in wild-type and mutant enzymes. The trafficking of oxygen atoms at the molybdenum center during catalysis will be followed by pulsed EPR studies in water enriched in 17-O. The electron transfer reactions at the molybdenum center will be investigated by protein film voltammetry, and the factors that control the rates of intramolecular electron transfer between the molybdenum and iron center in native and mutant sulfite oxidase will be investigated by flash photolysis and theoretical modeling.